DNA polymerase enzymes are naturally-occurring intracellular enzymes, and are used by a cell to replicate a nucleic acid strand using a template molecule to manufacture a complementary nucleic acid strand. Enzymes having DNA polymerase activity catalyze the formation of a bond between the 3' hydroxyl group at the growing end of a nucleic acid primer and the 5' phosphate group of a nucleotide triphosphate. These nucleotide triphosphates are usually selected from deoxyadenosine triphosphate (A), deoxythymidine triphosphate (T), deoxycytidine triphosphate (C) and deoxyguanosine triphosphate (G). However, DNA polymerases may incorporate modified or altered versions of these nucleotides. The order in which the nucleotides are added is dictated by base pairing to a DNA template strand; such base pairing is accomplished through "canonical" hydrogen-bonding (hydrogen-bonding between A and T nucleotides and G and C nucleotides of opposing DNA strands), although non-canonical base pairing, such as G:U base pairing, is known in the art. See e.g., Adams et al., The Biochemistry of the Nucleic Acids 14-32 (11th ed. 1992).
The in-vitro use of enzymes having DNA polymerase activity has in recent years become more common in a variety of biochemical applications including cDNA synthesis and DNA sequencing reactions (see Sambrook et al., (2nd ed. Cold Spring Harbor Laboratory Press, 1989) hereby incorporated by reference herein), and amplification of nucleic acids by methods such as the polymerase chain reaction (PCR) (Mullis et al., U.S. Pat. Nos. 4,683,195, 4,683,202, and 4,800,159, hereby incorporated by reference herein) and RNA transcription-mediated amplification methods (e.g., Kacian et al., PCT Publication No. WO91/01384 which enjoys common ownership with the present application and is hereby incorporated by reference herein).
Methods such as PCR make use of cycles of primer extension through the use of a DNA polymerase activity, followed by thermal denaturation of the resulting double-stranded nucleic acid in order to provide a new template for another round of primer annealing and extension. Because the high temperatures necessary for strand denaturation result in the irreversible inactivations of many DNA polymerases, the discovery and use of DNA polymerases able to remain active at temperatures above about 37.degree. C. to 42.degree. C. (thermostable DNA polymerase enzymes) provides an advantage in cost and labor efficiency. Thermostable DNA polymerases have been discovered in a number of thermophilic organisms including, but not limited to Thermus aquaticus, Thermus thermophilus, and species of the Bacillus. Thermococcus, Sulfobus, Pyrococcus genera.
DNA polymerases can be purified directly from these thermophilic organisms. However, substantial increases in the yield of DNA polymerase can be obtained by first cloning the gene encoding the enzyme in a multicopy expression vector by recombinant DNA technology methods, inserting the vector into a host cell strain capable of expressing the enzyme, culturing the vector-containing host cells, then extracting the DNA polymerase from a host cell strain which has expressed the enzyme.
The bacterial DNA polymerases that have been characterized to date have certain patterns of similarities and differences which has led some to divide these enzymes into two groups: those whose genes contain introns--intervening non-coding nucleotide sequences--(Class B DNA polymerases), and those whose DNA polymerase genes are roughly similar to that of E. coli DNA polymerase I and do not contain introns (Class A DNA polymerases).
By "non-coding" is meant that the nucleotides comprising both nucleic acid strands in such sequences do not contain 3-nucleotide codons that encode and correspond to amino acid residues of the mature protein. Introns are most often found in the genes of eukaryotic higher organisms but have also been found in lower organisms such as archaebacteria.
Several Class A and Class B thermostable DNA polymerases derived from thermophilic organisms have been cloned and expressed. Among the class A enzymes: Lawyer, et al., J. Biol. Chem. 264:6427-6437 (1989) and Gelfund et al., U.S. Pat. No. 5,079,352, report the cloning and expression of a full length thermostable DNA polymerase derived from Thermus aquaticus (Taq). Lawyer et al., in PCR Methods and Applications, 2:275-287 (1993), and Barnes, PCT Publication No. WO92/06188 (1992), disclose the cloning and expression of truncated versions of the same DNA polymerase, while Sullivan, EPO Publication No. 0482714A1 (1992), reports cloning a mutated version of the Taq DNA polymerase. Asakura et al., J. Ferment. Bioeng. (Japan), 74:265-269 (1993) have reportedly cloned and expressed a DNA polymerase from Thermus thermophilus. Gelfund et al., PCT Publication No. WO92/06202 (1992), have disclosed a purified thermostable DNA polymerase from Thermosipho africanus. A thermostable DNA polymerase from Thermus flavus was reported by Akhmetzjanov and Vakhitov, Nucleic Acids Res., 20:5839 (1992). Uemori et al., J. Biochem. 113:401-410 (1993) and EPO Publication No. 0517418A2 (1992) have reported cloning and expressing a DNA polymerase from the thermophilic bacterium Bacillus caldotenax. Ishino et al., Japanese Patent Application No. HEI 4[1992]-131400 (publication date Nov. 19, 1993) report cloning a DNA polymerase from Bacillus stearothermophilus.
Among the Class B enzymes: A recombinant thermostable DNA polymerase from Thermococcus litoralis was reported by Comb et al., EPO Publication No. 0 455 430 A3 (1991), Comb et al., EPO Publication No. 0547920A2 (1993), and Perler et al., Proc. Natl. Acad. Sci. (USA), 89:5577-5581 (1992). A cloned thermostable DNA polymerase from Sulfolobus solofatarius is disclosed in Pisani et al., Nucleic Acids Res. 20:2711-2716 (1992) and in PCT Publication WO93/25691 (1993). The thermostable enzyme of Pyrococcus furiosus is disclosed in Uemori et al., Nucleic Acids Res., 21:259-265 (1993), while a recombinant DNA polymerase was derived from Pyrococcus sp., as disclosed in Comb et al., EPO Publication No. 0547359A1 (1993).
By "thermostable" is meant that the enzyme remains has an optimal temperature of activity at a temperature greater than about 37.degree. C. to 42.degree. C. Preferrably, the enzymes of the present invention have an optimal temperature for activity of between about 50.degree. C. and 75.degree. C.; most preferably between 55.degree. C. and 70.degree. C., and most preferably between 60.degree. C. and 65.degree. C.
Many thermostable DNA polymerases possess activities additional to a DNA polymerase activity; these may include a 5'-3' exonuclease activity and/or a 3'-5' exonuclease activity. The activities of 5'-3' and 3'-5' exonucleases are well known to those of ordinary skill in the art. The 3'-5' exonuclease activity improves the accuracy of the newly-synthesized strand by removing incorrect bases that may have been incorporated; DNA polymerases in which such activity is low or absent, reportedly including Tag DNA polymerase, (see Lawyer et al., J. Biol Chem. 264:6427-6437), are prone to errors in the incorporation of nucleotide residues into the primer extension strand. In applications such as nucleic acid amplification procedures in which the replication of DNA is often geometric in relation to the number of primer extension cycles, such errors can lead to serious artifactual problems such as sequence heterogeneity of the nucleic acid amplification product (amplicon). Thus, a 3'-5' exonuclease activity is is a desired characteristic of a thermostable DNA polymerase used for such purposes.
By contrast, the 5'-3' exonuclease activity often present in DNA polymerase enzymes is often undesired in a particular application since it may digest nucleic acids, including primers, that have an unprotected 5' end. Thus, a thermostable DNA polymerase with an attenuated 5'-3' exonuclease activity, or in which such activity is absent, is also a desired characteristic of an enzyme for biochemical applications. Various DNA polymerase enzymes have been described where a modification has been introduced in a DNA polymerase which accomplishes this object. For example, the Klenow fragment of E. coli DNA polymerase I can be produced as a proteolytic fragment of the holoenzyme in which the domain of the protein controlling the 5'-3' exonuclease activity has been removed. The Klenow fragment still retains the polymerase activity and the 3'-5' exonuclease activity. Barnes, supra, and Gelfund et al., U.S. Pat. No. 5,079,352 have produced 5'-3' exonuclease-deficient recombinant Taq DNA polymerases. Ishino et al., EPO Publication No. 0517418A2, have produced a 5'-3' exonuclease-deficient DNA polymerase derived from Bacillus caldotenax.
Preparation of antisera or moloclonal antibodies to particular DNA polymerase enzymes has been described and is well known in the art. For example, Hu et al., J. Virol. 60:267-274 (1986) report specific immunoprecipiation of cloned reverse transcriptase and fusion proteins from Moloney Murine Leukemia Virus expressed in E. coli by recovering PAGE-separated MMLV reverse transcriptase from the gel, immunizing rabbits with the purified protein, and recovering the antisera. Livingston et al., Virology 50:388-395 (1972) disclose affinity chromatography of Avian Type C Viral transcriptase using antibodies able to differentiate between viral and host cell DNA polymerases. Spadari and Weissbach, J. Biol. Chem. 249:5809-5815 (1974) report that HeLa-derived DNA polymerase is not inhibited by antisera prepared against reverse transcriptases obtained from either the Mason-Pfizer monkey virus, the Wooley monkey virus, or the Rauscher murine leukemia virus. These publications are hereby incorporated herein by reference.